As described in U.S. patent application Ser. No. 10/935,986 filed Sep. 8, 2004 by Steven Greelish entitled “Method and Apparatus For Performing an Ultrasonic Survey,” incorporated herein by reference, a system is disclosed for use in hull surveying in which a network of acoustic elements is placed on the ocean floor and in which the exact position of a fiducial point on the hull is established.
In one embodiment of this system a neutrally buoyant robot performing as a remotely operated vehicle (ROV) is flown about the hull taking hull thickness measurements at very precise positions on the hull. It is a requirement of this system that the system be able to ascertain location with an error of less than one centimeter, which must be maintained continuously so as not to affect the hull thickness measurements. This requires exact positioning of the robot and thus the survey sensor. The precise positioning is made possible through the use of active acoustic ranging units and pingers on the hull that provide an instantaneous update of the position of the hull relative to the network of acoustic ranging apparatus on the ocean floor and thus the position of the robot relative to the hull.
Oftentimes, however, objects in the water interfere with the acoustic signals from the network as, for instance, when divers or other objects block these signals. When signals are blocked, an inertial navigation unit within the remotely operated vehicle takes over and estimates the position of the robot based on course direction and attitude. Course and attitude are derived from the outputs of an electronic compass, rate or laser gyros or other position sensors so that, in the case that acoustic positioning signals are not available, the ROV can estimate for itself its own position with a high degree of accuracy.
While the remotely operated vehicle normally must rely on an interactive acoustic positioning system to navigate, when this positioning system drops out temporarily or becomes otherwise unavailable, the vehicle must be able to continue to be navigated or to navigate autonomously until the positioning system again becomes available. This is typically accomplished through the use of an inertial guidance system consisting of gyros, electronic compasses, accelerometers, or some combination of the above. The internal guidance system computes position knowing the course of the vehicle, its speed and its attitude. While speed can be ascertained using accelerometers and other traditional speed-measuring apparatus, and whereas attitude measurements rely on the earth's gravitational field, course relies on the earth's magnetic field.
The problem of ascertaining accurate position upon acoustic network failure becomes acute due to the presence of ferromagnetic objects such as ship hulls, and fluctuating magnetic fields caused by electric motors. When a remotely operated vehicle is near an object such as a ship's hull or a wall containing steel or other ferromagnetic material, the navigation compass for the remotely operated vehicle has a heading that will be affected by the magnetic field generated by the object or flux concentration of the earth's magnetic field. Fluctuating magnetic fields generated by nearby electric motors such as the ones used by the propulsion system of the remotely operated vehicle itself will also affect the guidance system of the remotely operated vehicle.
Unlike the earth's magnetic field, which is uniform over a small area, fields generated by ferromagnetic objects exhibit a gradient of 1/r6 over a small distance, with the field being stronger nearer to the source and weaker the farther away the compass is from the source.
While in the past there have been a great number of static magnetic compensation systems, such as that described in U.S. Pat. No. 6,173,501, compensation of compasses in such static environments is not applicable to dynamically changing situations in which a compass is not at a fixed distance from a ferromagnetic source. This is precisely the case when a robot is flying around a steel hull.
For static compass compensation some sort of mechanical means is used, usually small permanent magnets and/or soft iron blocks located around the compass to correct for known magnetic fields. However, in the situation where either divers or remotely operated vehicles operate near a ship hull, there needs to be a dynamic magnetic anomaly compensation system that can provide heading accuracies unperturbed by the near field magnetic anomalies due to ferromagnetic objects or the aforementioned magnetic fields caused by electric motors.
By way of background, solid-state sensors have been used to detect magnetic fields. These sensors find applications in many major industries due to their relative low cost, small size and low power requirements. These devices have advantages over flux gate, SQUID, or other complicated low field sensing techniques. Solid-state magnetic sensors directly convert the strength of a magnetic field to a voltage or resistance and can be arranged in a small package to provide for detection along multiple axes in a small space. Such devices have been constructed using giant magneto resistance (GMR) materials and are one category of solid-state sensor.
GMR devices exhibit a change in the resistance when exposed to a magnetic field. The change in resistance is dependent on the orientation of the magnetic field relative to the magnetization of the GMR resistor itself. When the applied magnetic field is parallel to the magnetization of the GMR resistor, the resistance is minimum. When the applied magnetic field is antiparallel to the magnetization of the RMR resistor, the resistance is maximum. Such low field magnetic sensing with GMR sensors is described in an article presented at Sensors Expo—Baltimore, May 1999 by Carl H. Smith and Robert W. Schneider of Non-Volatile Electronics, Inc. of Eden Prairie, Minn.
More particularly, in dealing with magnetic bearings underwater, if one is in open water one can rely on the earth's magnetic field to obtain a reasonably reliable course line with the appropriate compensation for magnetic deviation.
However, when one is working in and around hulls and pipelines, the large quantities of steel and ferrous materials modify the magnetic force lines or actually introduce their own force lines, thereby disrupting or causing major errors in magnetic heading.
Such major errors in magnetic heading are exceptionally troublesome when considering a diver module that operates using an underwater compass system. When operating near any steel structure the underwater compass systems become wildly inaccurate. Moreover, being able to compensate for such magnetic heading anomalies is critical in vehicle control systems, especially for underwater surveying.
While, as mentioned above, when one has an acoustic positioning system that handles vehicle position and attitude while in the line of sight to the acoustic network, these networks can be blocked momentarily when, for instance, a diver gets in the way. At this point the vehicle is without its precise positioning control system.
In order to handle these periods of outages, one can attempt to use an inertial management unit or IMU. Traditionally, inertial management units have used multiple sensors fused to make estimates as to the exact location of the vehicle. In order to do so, gyroscopes and especially laser gyros are used in such systems. However, these gyros suffer drift and precession.
Although by mixing magnetic sensors and gyros, be they laser gyros or rate gyros, one is attempting to make better estimations of the actual position of the vehicle, drift and precession quickly degrade heading measurements. One of course can use a magnetic sensor to correct for drift and precession, if one knows how good the magnetic sensor is.
The problem as mentioned above is that when one is working close to a hull, the magnetic sensor is not of much use. As a result, precession becomes cumulative and the vehicle's course as determined by the inertial management unit deteriorates badly over time.
If the magnetic field relative to the compass is static, one can use the above-mentioned mechanical and electrical compensation systems. It is noted that, whether one considers the diver application or a remotely operated vehicle that is flying around a hull, the relative position of the compass and the ship hull vary. This leads to a dynamic situation in which static compensation is inapplicable.
There is therefore a unique need, especially in surveying systems, for the ability to compensate compasses used in inertial navigation systems for magnetic anomalies that vary dynamically depending on the distance of compass to a ferromagnetic object or depending on the closeness and field strength of an electric motor.